Nickel sponge catalysts generally consist of milled polymorphic particles up to 20 micrometers in size. These particles are commonly sold in drums containing alkaline water and nickel at a weight ratio of approximately 90:85 (Ni:water). The catalyst must be contained in water as it is pyrophoric and can spontaneously combust if allowed to dry out in air. Such a catalyst in water system is known as a ‘nickel slurry’.
Other compounds of nickel may be sold as catalysts for example nickel oxides, nickel halides, nickel complexes and nickel carbonyls but these are not generally provided as slurries.
Not all nickel compounds (including those listed above) will be suitable for use as a catalyst since certain properties of any material are key as to whether it is able to perform as a catalyst. Such properties include the requirement that the material has a clean surface and a high surface area thereby to enable the reactants to access the surface of the catalyst.
Nickel particles in a nickel slurry tend to settle within the container in which they are housed, e.g. a drum, due to the high density of nickel compared to water (density of nickel=9 g cm−3, density of water=1 g cm−3). These particles may even form a dense cake of sediment. Banging and vibration of the nickel slurry container, such as might occur during transportation, appears to make the cake more dense. The settling of the nickel catalyst makes it difficult to pour or remove from the drum, this poor flow behaviour being especially problematic due to the pyrophoric nature of the dry catalyst.
WO 2006/063600 relates to adjusting catalysts' and catalyst precursors' suspension and settling properties using rheology modifiers, and concentrates on using rheology modifiers that speed up the rate of settling with nickel catalysts.